The invention relates to a filter arrangement comprising a balanced amplifier having an inverting input, a non-inverting input, an inverting output and a non-inverting output, at least a first input circuit arranged between a first input terminal and the inverting input, at least a second input circuit arranged between a second input terminal and the non-inverting input, a first feedback circuit arranged between the non-inverting output and the inverting input, and a second feedback circuit arranged between the inverting output and the non-inverting input, which first and second input circuit each comprise a substantially identical first circuit element, which first and second feedback circuits each comprise a substantially identical second circuit element, one of said two circuit elements being a capacitor and the other circuit element being a resistor and the magnitude of at least one of said two circuit elements being adjustable.
Such a filter arrangement may be employed in, for example filters, oscillators and resonators.
Such a filter arrangement is known from U.S. Pat. No. 4,509,019. In said arrangement one circuit element is constituted by a capacitor having a fixed capacitance and the other circuit element is constituted by a resistor formed by a MOS transistor whose resistance value can be adjusted by means of a control voltage applied to its gate electrode. By means of this control voltage the RC-time constant of the resistor and the capacitor can be adjusted to the desired value. The capacitors may be arranged in the feedback circuits and the resistors in the input circuits or the capacitors may be arranged in the input circuits and the resistors in the feedback circuits. In the first case the filter circuit constitutes an integrator or low-pass filter and in the second case it constitutes a differentiator or high-pass filter.
A balanced input voltage is applied to the first and the second input terminal of the filter arrangement and a balanced output voltage is available on the inverting and the non-inverting output of the amplifier. This output voltage is substantially free of even-order harmonic distortion, which is inter alia caused by non-linearities of the MOS resistors. This is because these harmonic components cancel one another as a result of the symmetrical construction of the arrangement and the balanced drive.
A drawback of this known arrangement is that it can be realized only by means of a semiconductor process enabling MOS transistors to be fabricated. The arrangement cannot be realized by means of a purely bipolar process. Therefore, it is the object of the invention to provide a balanced filter arrangement with a variable RC-time constant which can also be fabricated by means of a bipolar process.